Surface bonding of ceramic bodies

ABSTRACT

Ceramic bodies are bonded together via a layer of an oxidation reaction product of a molten metal, which metal is present in one or both of the ceramic bodies prior to bonding. At least one of the ceramic bodies comprises a ceramic product formed by the oxidation reaction of molten parent metal (e.g., alumina from molten aluminum) and grown as molten metal is transported through, and oxidized on the surface of, its own oxidation product. One or both of the ceramic bodies used in the bonding process contains surface-accessible channels of residual metal, i.e., metal channels which have resulted from molten-metal transport during the ceramic growth process. When the suitably assembled ceramic bodies are heated in the presence of an oxidant at a temperature above the melting point of the residual metal, molten metal at the surface of at least one of the ceramic bodies reacts with the oxidant to form a layer of oxidation reaction product, which may or may not incorporate at least one filler material. This layer of oxidation reaction product continues to grow between the facing surfaces of the assembled ceramic bodies until the oxidation reaction product forms a bond between the ceramic bodies.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of copending application(s) Ser. No. 07/591,623filed on Oct. 2, 1990 which issued on Apr. 14, 1992, as U.S. Pat. No.5,104,835, which is a continuation of U.S. Ser. No. 327,022, filed onMar. 22, 1989, and issued on Oct. 2, 1990, as U.S. Pat. No. 4,960,736,which is a continuation-in-part of U.S. Ser. No. 039,510, filed Apr. 17,1987, now U.S. Pat. No. 4,824,008, that in turn is acontinuation-in-part of U.S. Ser. No. 907,930, filed Sep. 16, 1986, nowabandoned. U.S. application Ser. No. 039,510, issued to U.S. Pat. No.4,824,008, on Apr. 25, 1989.

BACKGROUND OF THE INVENTION

The present invention relates to a method of bonding the planar orotherwise congruent ceramic faces of abutting bodies such as plates,disks, or the like.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method ofbonding the congruent surfaces of ceramic bodies of which at least onebody is a polycrystalline ceramic material comprising the oxidationreaction product of a parent metal with an oxidant, and havinginterconnected metallic constituents derived at least in part from theparent metal, and optionally one or more filler materials, as describedbelow in detail.

For this ceramic body, the polycrystalline ceramic material isinterconnected in three dimensions and the interconnected metal,distributed through at least a portion of the ceramic body, is at leastpartially open or accessible or rendered accessible, from at least onebonding surface. Said bonding surface of the ceramic body can now bebonded to a congruent surface of an abutting ceramic body.

In the method of this invention, the two ceramic bodies to be bonded(e.g., two oxidation reaction products as described above, or oneproduct which is an oxidation reaction product as described above andanother ceramic product made by known or conventional techniques otherthan by the oxidation of a molten parent metal) are assembled so thatthe surfaces to be bonded substantially abut, although there may be aslight separation as explained below. In one embodiment, the assembledceramic bodies are heated in an oxidizing atmosphere at a temperatureabove the melting point of the interconnected metal, but below themelting point of the oxidation reaction product, and on reaction, anoxidation reaction product is grown between the abutting surfacescausing them to bond together.

Other embodiments of the instant invention include using a solidoxidant, a liquid oxidant, or both, disposed between the two ceramicbodies to be bonded. These solid or liquid oxidants could be utilized inthe form of a layer interposed between the ceramic bodies or they may bedisposed within the surface porosity of one or both of the abuttingceramic faces which are to be bonded. Further, these solid and liquidoxidants may be used by themselves or in combination with a vapor-phaseoxidant. In a preferred embodiment of the instant invention, at leastone filler material is disposed between the ceramic bodies to be bondedor within the porosity of one or both of the abutting ceramic faceswhich are to be bonded. This embodiment provides a ceramic compositebond between the ceramic bodies.

Generally, in accordance with the present invention, there is provided amethod of bonding ceramic bodies along substantially congruent surfacesthereof, the method comprising the following steps. There is provided afirst body of ceramic comprising a ceramic product formed by theoxidation reaction of molten parent metal, e.g., aluminum, and anoxidant, e.g., air, and grown as molten metal is transported through,and oxidized on the surface of, its own oxidation reaction product. Thisfirst ceramic body comprises a polycrystalline oxidation reactionproduct, e.g., alumina, and interconnected residual metal, e.g.,aluminum, and optionally may comprise a composite formed by infiltratinga filler with the oxidation reaction product. The first body of ceramicis assembled adjacent to a second body of ceramic in a manner such thata pair of surfaces of the first and second bodies to be bonded togetherface one another. The assembled ceramic bodies are then heated in thepresence of an oxidant at a temperature above the melting point of theresidual metal to induce transport of the residual metal toward thebonding surfaces where oxidation reaction product continues to grow asdescribed above, thereby effecting a bond between the first and secondbodies.

As used in this specification and the appended claims, the terms beloware defined as follows:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body contains minor or substantialamounts of one or more metallic constituents and/or porosity(interconnected and isolated) most typically within a range of fromabout 1-40% by volume, but may be higher.

"Oxidation reaction product" generally means one or more metals in anyoxidized state wherein the metal has given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of reaction of one or more metals with an oxidantsuch as those described herein.

"Oxidant" as used herein in conjunction with ceramic matrix compositebodies means one or more suitable electron acceptors or electron sharersand may be a solid, a liquid or a gas or some combination of these(e.g., a solid and a gas) at the oxidation reaction conditions. Typicaloxidants include, without limitation, oxygen, nitrogen, a halogen,sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium, and orcompounds and combinations thereof, for example, silica or silicates (asa source of oxygen), methane, ethane, propane, acetylene, ethylene,propylene (the hydrocarbon as a source of carbon), and mixtures such asair, H2/H2O and CO/CO2 (source of oxygen), the latter two (i.e., H2/H2Oand CO/CO2) being useful in reducing the oxygen activity of theenvironment.

"Parent metal" is intended to refer to relatively pure metals,commercially available metals with impurities and/or alloyingconstituents therein, and alloys and intermetallic compounds of themetals. When a specific metal is mentioned, the metal identified shouldbe read with this definition in mind unless indicated otherwise by thecontext.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view, partially in cross-section, showing anassembly of a first and second ceramic body and a barrier means, inaccordance with one embodiment of the present invention; and

FIG. 2 is a schematic view, partially in cross-section, showing anassembly of a first and second ceramic body, a reservoir metal body, anda barrier means, in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A first ceramic body is produced by the method as disclosed in U.S. Pat.No. 4,713,360 which issued on Dec. 15, 1987 and was based on U.S. patentapplication Ser. No. 818,943, filed Jan. 15, 1986, to Marc S. Newkirk,et al, and entitled "Novel Ceramic Materials and Methods of MakingSame". According to the method, a parent metal precursor, e.g.,aluminum, is heated in the presence of a vapor-phase oxidant, e.g. air,to a temperature above its melting point, but below the melting point ofthe oxidation reaction product, to form a body of molten parent metal.The molten parent metal is reacted with the vapor-phase oxidant to forman oxidation reaction product, which product is maintained at leastpartially in contact with, and extends between, the body of moltenparent metal and the vapor-phase oxidant. In this temperature range,molten parent metal is transported through the previously formedoxidation reaction product, towards the vapor-phase oxidant. As themolten parent metal contacts the vapor-phase oxidant at the interfacebetween the vapor-phase oxidant and previously formed oxidation reactionproduct, it is oxidized by the vaporphase oxidant, and thereby grows orforms a progressively thicker layer or body of oxidation reactionproduct. The process is continued for a time sufficient to produce aceramic body having interconnected metallic constituents includingnonoxidized parent metal. This metal is at least partially open oraccessible, or can be rendered accessible by fracturing, machining, etc.This ceramic body is hereinafter identified as the "first ceramic body".The process may be enhanced by the use of an alloyed dopant, such as inthe case of an aluminum parent metal oxidized in air. These dopantsinitiate, accelerate, enhance or promote the formation of channels formetal transport within the polycrystalline material. The dopants whichmake this metallic transport possible are, as in the case of aluminum,alloyed into the parent metal. A single dopant material may be used, ora combination of dopants may be used, and in varying concentrations andproportions, depending upon such factors as parent metal and processconditions.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium metal and zinc metal,preferably in combination with each other or singly or together incombination with other dopant(s) described below. These metals, or asuitable source of the metals, are alloyed into the aluminum-basedparent metal at temperatures preferably below about 900° C., and may beat concentrations for each of between about 0.1-10% by weight based onthe total weight of the resulting doped metal. Concentrations within theappropriate range for magnesium and zinc appear to promote the ceramicgrowth, enhance metal transport and favorably influence the growthmorphology of the resulting oxidation reaction product.

Other dopants which are effective in promoting polycrystalline oxidationreaction product growth for aluminum-based parent metal systems are, forexample, silicon, germanium, tin and lead, especially when used incombination with magnesium or zinc. One or more of these other dopants,or a suitable source of the desired dopant or dopants, is alloyed intothe aluminum parent metal system to produce a concentration for any onesuch dopant of from about 0.5 to about 15% by weight of the total alloy;however, more desirable growth kinetics and growth morphology areobtained with a dopant concentration in the range of from about 1-10% byweight of the total parent metal alloy. Lead as a dopant is generallyalloyed into the aluminum based parent metal at a temperature of atleast 1000° C. so as to make allowances for its low solubility inaluminum; however, the addition of other alloying components, such astin, will generally increase the solubility of lead and allow thealloying materials to be added at a lower temperature. One or moredopants may be used depending upon the circumstances, as explainedabove. For example, in the case of aluminum with air as the oxidant,particularly useful combinations of dopants include (a) magnesium andsilicon or (b) magnesium, zinc and silicon. In such examples, apreferred magnesium concentration falls within the range of from about0.1 to about 3% by weight, for zinc in the range from about 1 to about6% by weight, and for silicon in the range of from about 1 to about 10%by weight.

Additional examples of dopant materials, useful with an aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttrium,which may be used individually or in combination with one or more otherdopants depending on the oxidant and process conditions. Sodium andlithium may be used in very small amounts, even in the parts per millionrange, as low as about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praeseodymium, neodymium andsamarium are also useful dopants, and here again especially when used incombination with other dopants.

The method of U.S. Pat. No. 4,713,360 was improved by the use ofexternal dopants applied to the surface of the precursor metal asdisclosed in commonly owned U.S. Pat. No. 4,853,352, which issued onAug. 1, 1989, from U.S. Pat. application Ser. No. 220,935, filed on Jun.23, 1988, which was a Continuation of U.S. Pat. application Ser. No.822,999 filed Jan. 27, 1986, and now abandoned in the names Marc S.Newkirk, et al., and entitled "Methods of Making Self-Supporting CeramicMaterials", which was a continuation-in-part of U.S. patent applicationSer. No. 776,965, filed Sep. 17, 1985, and now abandoned which was acontinuation-in-part of U.S, patent application Ser. No. 747,788, filedJun. 25, 1985, and now abandoned which was a continuation-in-part ofU.S. patent application No. 632,636, filed on Jul. 20, 1984, and nowabandoned all entitled "Methods of Making Self-Supporting CeramicMaterials" and filed in the names of Marc S. Newkirk et al.

When dopants are applied externally, useful dopants for an aluminumparent metal, particularly with air as the oxidant, include, forexample, magnesium and zinc, either singly or in combination with eachother or together in combination with other dopant(s) described below.One or more or all of these dopants, or one or more or all of suitablesources of these dopants, are applied externally to the aluminum-basedparent metal either in elemental form or more preferably as a compound,e.g., MgO or ZnO. Zinc, if applied as an external dopant to aluminum,may not require the presence of magnesium to operate effectively.

Other dopants which are effective in promoting polycrystalline oxidationreaction product growth for aluminum-based parent metal systems are, forexample, silicon, germanium, tin and lead, especially when used incombination with magnesium or zinc. At least one of the other dopants ora suitable source of the dopant, is applied externally to the parentmetal and, optionally, at least one of the remaining dopants or sourcesthereof is alloyed into the aluminum parent metal system.

Additional examples of dopant materials useful with aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttrium,which may be used individually or in combination with one or more otherdopants depending on the oxidant and process conditions. Sodium andlithium may be used in very small amounts, even in parts per millionrange, as low as about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful as dopants, and here again especially when used incombination with other dopants.

It is not necessary that all of the dopants be applied to an externalsurface of the parent metal. Thus, one or more of the dopants may beinternally alloyed with or otherwise incorporated into the parent metal,and the other dopant or dopants may be externally applied to the parentmetal surface, in a with the invention described in U.S. Pat. No.4,853,352; Additionally, dopants alloyed within the parent metal may beaugmented by externally applied dopants. For example, concentrationdeficiencies of one or both of internal or alloyed dopants may beaugmented by externally applied dopants. In the case of aluminum, theremay be no common commercially available alloys which are optimallyconstituted with respect to internally alloyed concentrations ofmaterials which may serve as dopant materials. It has been found thatsuch alloys may be used by externally applying selected dopant(s) to asurface of such metal.

Preferably, the dopant materials are applied to a portion of a surfaceof the parent metal as a uniform coating thereon which is thin relativeto the thickness of the body of parent metal to which it is applied. Thequantity of dopant is effective over a wide range relative to the amountof parent metal to which it is applied and, in the case of aluminum,experiments have established a wide range of operable limits. Forexample, when utilizing silicon in the form of silicon dioxide as adopant for an aluminum-based parent metal using air or oxygen as theoxidant, quantities as low as about 0.00003 gram of silicon per gram ofparent metal, or about 0.0001 gram of silicon per square centimeter ofexposed parent metal surface, together with a second dopant such as amagnesium source produce the ceramic growth phenomenon. It has also beenfound that a ceramic structure is achievable from an aluminum-basedparent metal containing silicon using air as the oxidant, by applying tothe surface MgO dopant in an amount greater than about 0.0008 gram Mgper gram of parent metal to be oxidized and greater than about 0.003gram Mg per square centimeter of parent metal surface upon which the MgOis applied.

Thus, generally, a dopant may be used in conjunction with the parentmetal in forming the first ceramic body.

Commonly owned U.S. Pat. No. 4,851,375, which issued on Jul. 25, 1989,from U.S. Pat. application Ser. No. 819,397, filed Jan. 17, 1986, in thenames of Marc S. Newkirk et al., which was a continuation-in-part ofU.S. patent application Ser. No. 697,876, filed Feb. 4, 1985 and nowabandoned in the names of Marc S. Newkirk et al., and both entitled"Composite Ceramic Articles and Methods of Making Same," discloses anovel method for producing self-supporting ceramic composites by growingan oxidation reaction product from a parent metal into a permeable massof filler, thereby infiltrating the filler with a ceramic matrix.

The parent metal, which, for example, may comprise aluminum, silicon,zirconium, tin or titanium, and a permeable mass of filler material arepositioned adjacent to each other and oriented with respect to eachother so that a direction of growth of the oxidation reaction productwill be towards the filler material, and the oxidation reaction productwill permeate or engulf at least a portion of the filler material suchthat void space between filler particles or articles will be filled inby the grown oxidation reaction product matrix.

Examples of useful fillers, depending upon parent metal and oxidationsystems chosen, include one or more of aluminum oxide, silicon carbide,silicon aluminum oxynitride, zirconium oxide, zirconium boride, titaniumnitride, barium titanate, boron nitride, silicon nitride, ferrousalloys, e.g., iron-chromium-aluminum alloy, carbon, aluminum andmixtures thereof. However, any suitable filler may be employed, andthree specific classes of useful fillers may be identified.

The first class of fillers contains those chemical species which, underthe temperature and oxidizing conditions of the process, are notvolatile, are thermodynamically stable and do not react with or dissolveexcessively in the molten parent metal. Numerous materials are known tothose skilled in the art as meeting such criteria in the case wherealuminum parent metal and air or oxygen as the oxidant are employed.Such materials include the single-metal oxides of: aluminum, Al₂ O₃ ;cerium, CeO₂ ; hafnium, HfO₂ ; lanthanum, La₂ O₃ ; neodymium, Nd₂ O₃ ;praseodymium, various oxides, samarium, Sm₂ O₃ ; scandium, Sc₂ O₃ ;thorium, ThO₂ ; uranium, UO₂ ; yttrium, Y₂ O₃ ; and zirconium, ZrO₂. Inaddition, a large number of binary, ternary, and higher order metalliccompounds such as magnesium aluminate spinel, MgO Al₂ O₃ are containedin this class of stable refractory compounds.

The second class of suitable fillers are those which are notintrinsically stable in the oxidizing and high temperature environmentof the process, but which, due to relatively slow kinetics of thedegradation reactions, can be incorporated as a filler phase within thegrowing ceramic body. An example in the case of an alumina ceramicmatrix is silicon carbide. This material would oxidize completely underthe conditions necessary to oxidize aluminum with or air in accordancewith the process described in U.S. Pat. No. 4,851,375, were it not for aprotective layer of silicon oxide forming and covering the siliconcarbide particles to limit further oxidation of the silicon carbide.

A third class of suitable fillers are those which are not, onthermodynamic or on kinetic grounds, expected to survive the oxidizingenvironment or exposure to molten metal necessary for practice of theinvention described in U.S. Pat. No. 4,851,375. Such fillers can be madecompatible with the process 1) if the oxidizing environment is made lessactive, or 2) through the application of a coating thereto, which makesthe species kinetically non-reactive in the oxidizing environment. Anexample of such a class of fillers would be carbon fiber employed inconjunction with a molten aluminum parent metal. If the aluminum isoxidized with air or oxygen at, for example, 1250° C., to generate amatrix incorporating the fiber, the carbon fiber will tend to react withboth the aluminum (to form aluminum carbide) and the oxidizingenvironment (to form CO or CO₂). These unwanted reactions may be avoidedby coating the carbon fiber (for example, with alumina) to preventreaction with the parent metal and/or oxidant and optionally employing aCO/CO₂ atmosphere as oxidant which tends to be oxidizing to the aluminumbut not to the carbon fiber.

When one or more dopant materials (described below) are required ordesirable to promote or facilitate growth of the oxidation reactionproduct, the dopant may be used on and/or in the parent metal and,alternatively or in addition, the dopant may be used on, or be providedby, the filler material. Certain parent metals under specific conditionsof temperature and oxidizing atmosphere meet the criteria necessary forthe oxidation phenomenon described in U.S. Pat. No. 4,851,375 with nospecial additions or modifications. However, as described in theaforesaid Commonly Owned Patent Applications and Patens, dopantmaterials used in combination with the parent metal can favorablyinfluence or promote the oxidation reaction process. While not wishingto be bound by any particular theory or explanation of the function ofthe dopants, it appears that some of them are useful in those caseswhere appropriate surface energy relationships between the parent metaland its oxidation reaction product do not intrinsically exist. Thus,certain dopants or combinations of dopants, which reduce thesolid-liquid interfacial energy, will tend to promote or accelerate thedevelopment of the polycrystalline structure formed upon oxidation ofthe metal into one containing channels for molten metal transport, asrequired for the new process. Another function of the dopant materialsmay be to initiate the ceramic growth phenomenon, apparently either byserving as a nucleating agent for the formation of stable oxidationproduct crystallites, or by disrupting an initially passive oxidationproduct layer in some fashion, or both. This latter class of dopants maynot be necessary to create the ceramic growth phenomenon of the presentinvention, but such dopants may be important in reducing any incubationperiod for the initiation of such growth to within commerciallypractical limits for certain parent metal systems.

The function or functions of the dopant material can depend upon anumber of factors other than the dopant material itself. These factorsinclude, for example, the particular parent metal, the end productdesired, the particular combination of dopants when two or more dopantsare used, the use of an externally applied dopant in combination with analloyed dopant, the concentration of the dopant, the oxidizingatmosphere, and the process conditions.

The dopant or dopants (1) may be provided as alloying constituents ofthe parent metal, (2) may be applied to at least a portion of thesurface of the parent metal, or (3) may be applied to or supplied by thefiller or a part of the filler bed, or any combination of two or moretechniques (1), (2) and (3) may be employed. For example, an alloyeddopant may be used in combination with an externally applied dopant. Inthe case of technique (3), where a dopant or dopants are applied to thefiller, the application may be accomplished in any suitable manner, suchas by dispersing the dopants throughout part of the entire mass offiller in fine-droplet or particulate form, preferably in a portion ofthe bed of filler adjacent the parent metal. Application of any of thedopants to the filler may also be accomplished by applying a layer ofone or more dopant materials to and within the bed, including any of itsinternal openings, interstices, passageways, intervening spaces, or thelike, that render it permeable. A source of the dopant may also beprovided by placing a rigid body containing the dopant in contact withand between at least a portion of the parent metal surface and thefiller bed. For example, if a silicon dopant is required, a thin sheetof silicon-containing glass or other material can be placed upon asurface of the parent metal onto which a second dopant had beenpreviously applied. When the parent metal overlaid with thesiliconcontaining material is melted in an oxidizing environment (e.g.,in the case of aluminum in air, between about 850° C. to about 1450° C.,preferably about 900° C. to about 1350° C.), growth of thepolycrystalline ceramic material into the permeable filler occurs. Inthe case where the dopant is externally applied to at least a portion ofthe surface of the parent metal, the polycrystalline oxide structuregenerally grows within the permeable filler substantially beyond thedopant layer (i.e., to beyond the depth of the applied dopant layer). Inany case, one or more of the dopants may be externally applied to theparent metal surface and/or to the permeable bed of filler.Additionally, dopants alloyed within the parent metal and/or externallyapplied to the parent metal may be augmented by dopant(s) applied to thefiller bed. Thus, any concentration deficiencies of the dopants alloyedwithin the parent metal and/or externally applied to the parent metalmay be augmented by additional concentration of the respective dopant(s)applied to the bed, and vice versa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium metal and zinc metal, incombination with each other or in combination with other dopantsdescribed below. These metals, or a suitable source of the metals, maybe alloyed into the aluminum-based parent metal at concentrations foreach of between about 0.1-10% by weight based on the total weight of theresulting doped metal. The concentration range for any one dopant willdepend on such factors as the combination of dopants and the processtemperature. Concentrations within this range appear to promote theceramic growth, enhance metal transport and favorably influence thegrowth morphology of the resulting oxidation reaction product.

Other dopants which are effective in promoting polycrystalline oxidationreaction product growth, for aluminum-based parent metal systems are,for example, silicon, germanium, tin and lead, especially when used incombination with magnesium or zinc. One or more of these other dopants,or a suitable source of them, is alloyed into the aluminum parent metalsystem at concentrations for each of from about 0.5 to about 15% byweight of the total alloy; however, more desirable growth kinetics andgrowth morphology are obtained with dopant concentrations in the rangeof from about 1-10% by weight of the total parent metal alloy. Lead as adopant is generally alloyed into the aluminum-based parent metal at atemperature of at least 1000° C. so as to make allowances for its lowsolubility in aluminum; however, the addition of other alloyingcomponents, such as tin, will generally increase the solubility of leadand allow the alloying materials to be added at a lower temperature.

One or more dopants may be used depending upon the circumstances, asexplained above. For example, in the case of an aluminum parent metaland with air as the oxidant, particularly useful combinations of dopantsinclude (a) magnesium and silicon or (b) magnesium, zinc and silicon. Insuch examples, a preferred magnesium concentration falls within therange of from about 0.1 to about 3% by weight, for zinc in the range offrom about 1 to about 6% by weight, and for silicon in the range of fromabout 1 to about 10% by weight.

Additional examples of dopant materials, useful with an aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttrium,which may be used individually or in combination with one or more otherdopants depending on the oxidant and process conditions. Sodium andlithium may be used in very small amounts in the parts per millionrange, typically about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful dopants, and herein again especially when used incombination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all, or a portion of, the surface ofthe parent metal enables local ceramic growth from the parent metalsurface or portions thereof and lends itself to growth of thepolycrystalline ceramic material into the permable filler in selectedareas. Thus, growth of the polycrystalline ceramic material into thepermeable bed can be controlled by the localized placement of the dopantmaterial upon the parent metal surface. The applied coating or layer ofdopant is thin relative to the thickness of the parent metal body, andgrowth or formation of the oxidation reaction product into the permeablebed extends to substantially beyond the dopant layer, i.e., to beyondthe depth of the applied dopant layer. Such layer of dopant material maybe applied by painting, dipping, silk screening, evaporating, orotherwise applying the dopant material in liquid or paste form, or bysputtering, or by simply depositing a layer of a solid particulatedopant or a solid thin sheet or film of dopant onto the surface of theparent metal. The dopant material may, but need not, include eitherorganic or inorganic binders, vehicles, solvents, and/or thickeners.More preferably, the dopant materials are applied as powders to thesurface of the parent metal or dispersed through at least a portion ofthe filler. One particularly preferred method of applying the dopants tothe parent metal surface is to utilize a liquid suspension of thedopants in a water/organic binder mixture sprayed onto a parent metalsurface in order to obtain an adherent coating which facilitateshandling of the doped parent metal prior to processing.

The dopant materials when used externally are usually applied to aportion of a surface of the parent metal as a uniform coating thereon.The quantity of dopant is effective over a wide range relative to theamount of parent metal to which it is applied and, in the case ofaluminum, experiments have failed to identify either upper or loweroperable limits. For example, when utilizing silicon in the form ofsilicon dioxide externally applied as the dopant for an aluminum-basedparent metal using air or oxygen as the oxidant, quantities as low0.00003 gram of silicon per gram of parent metal together with a seconddopant providing a source of magnesium and/or zinc produce thepolycrystalline ceramic growth phenomenon. It also has been found that aceramic structure is achievable from an aluminum-based parent metalusing air or oxygen as the oxidant by using MgO as the dopant in anamount greater than 0.0008 gram of Mg per gram of parent metal to beoxidized and greater than about 0.003 gram of Mg per square centimeterof parent metal surface upon which the MgO is applied. It appears thatto some degree an increase in the quantity of dopant materials willdecrease the reaction time necessary to produce the ceramic composite,but this will depend upon such factors as type of dopant, the parentmetal and the reaction conditions.

As used in the specification and claims of Commonly Owned U.S. Pat. No.4,713,360, 4,853,352 and 4,851,375, "oxidation reaction product" meansone or more metals in any oxidized state wherein the metal(s) have givenup electrons to or shared electrons with another element, compound, orcombination thereof. Accordingly, an "oxidation reaction product" underthis definition includes the product of the reaction of one or moremetals with an oxidant such as oxygen, nitrogen, a halogen, sulphur,phosphorus, arsenic, carbon, boron, selenium, tellurium and compoundsand combinations thereof, for example, methane, ethane, propane,acetylene, ethylene, propylene and mixtures such as air, H₂ /H₂ O andCO/CO₂, the latter two (i.e., H₂ /H₂ O and CO/CO₂) being useful inreducing the oxygen activity of the environment.

The first ceramic body thus may comprise a composite formed byinfiltrating a filler with the oxidation reaction product.

A method for producing ceramic composite bodies having a geometry orshape is disclosed in the commonly owned and copending U.S. patentapplication Ser. No. 338,471, filed Apr. 14, 1989, as a Continuation ofU.S. Pat. No. 861,025, filed May 8, 1986, and now abandoned entitled"Shaped Ceramic Composites and Methods of Making the Same", and in thenames of Marc S. Newkirk et al. In accordance with the method of thisinvention, the developing oxidation reaction product infiltrates apermeable preform in the direction towards a defined surface boundary. Asolid or liquid oxidant may be used in conjunction with the vapor-phaseoxidant, and the preform is permeable to the gaseous oxidant and toinfiltration by the developing oxidation reaction product. The resultingceramic composite has the geometry of the preform. When a solid oxidantis employed, it may be dispersed through the entire preform or through aportion of the preform adjacent the parent metal, such as in particulateform and admixed with the preform, or it may be utilized as coatings onthe preform particles. Any suitable solid oxidant may be employeddepending upon its compatibility with the vapor-phase oxidant. Suchsolid oxidants may include suitable elements, such as boron or carbon,or suitable reducible compounds, such as silicon dioxide (as a source ofoxygen) or certain borides of lower thermodynamic stability than theboride reaction product of the parent metal.

If a liquid oxidant is employed, the liquid oxidant may be dispersedthroughout the entire preform or a portion thereof adjacent to themolten metal, provided such liquid oxidant does not prevent access ofthe vaporphase oxidant to the molten parent metal. Reference to a liquidoxidant means one which is a liquid under the oxidation reactionconditions, and so a liquid oxidant may have a solid precursor, such asa salt, which is molten or liquid at the oxidation reaction conditions.Alternatively, the liquid oxidant may have a liquid precursor, e.g., asolution of a material, which is used to coat part or all of the poroussurfaces of the preform and which is melted or decomposed at the processconditions to provide a suitable oxidant moiety. Examples of liquidoxidants as herein defined include low melting glasses.

The entire disclosures of all of the foregoing Commonly Owned PatentApplications and Patents are expressly incorporated herein by reference.

In the present method, a first ceramic body is bonded to another ceramicbody, either of like kind or of a different ceramic (hereinafter "secondceramic body") by the development of a bond layer derived from the firstceramic body as a result of the oxidation reaction of molten parentmetal contained in the first ceramic body. Two or more ceramic bodiescan be so bonded in a single operation provided that, at each pair offacing surfaces, at least one of the surfaces is a surface of the firstceramic body formed by the oxidation of molten parent metal, and grownas molten metal is transported through, and oxidized on the surface of,its own reaction product. The interconnected metal of the first ceramicbody is the source of metal required for the formation of the ceramicbonding layer. More particularly, the first ceramic containssurface-accessible residual metal present as a result of molten parentmetal transport during the ceramic growth process. In the case of a bondbetween two ceramic bodies of like kind, in that each ceramic bodycontains interconnected parent metal as described above, then bothceramic bodies may participate in the growth of the bond layer at theircommon interface.

The ceramic bodies are assembled with each pair of surfaces to be bondedfacing one another, either in intimate contact, or at a small standoffor separation. For example, a single pair of first and second ceramicbodies can be arrayed, or a first ceramic body can be assembled betweentwo second ceramic bodies. An array of multiple surfaces, such asplates, can be used, provided that at least every other surface or layeris a body of oxidation reaction product containing interconnected metal.

FIGS. 1 and 2 each show typical assemblies used in accordance with theinvention in which a first ceramic body 2 (FIG. 1) or 2' (FIG. 2) ispositioned with a surface thereof facing a corresponding surface of asecond ceramic body 4 (FIG. 1) or 4' (FIG. 2). The bonding layer will begrown between the facing surfaces to bond ceramic bodies 2 and 4 of FIG.1; and 2' and 4' of FIG. 2.

The facing surfaces can be essentially in contact with one another,provided that the oxidant required for the molten metal oxidation cancontact the surface of the first ceramic body. However, inasmuch as theoxidation reaction product of the bonding layer is able to grow bytransport of molten metal therethrough and oxidation of molten metaladjacent thereto (as has occurred in the formation of the first ceramicbody itself), an initial separation between facing surfaces can be used,provided that sufficient molten metal is available, and the processconditions maintained for a sufficient time, that the growth process cancontinue to the degree required for the pre-separated surfaces to bejoined. When such a standoff is used, it may be beneficial to provide asmall angle, e.g., of about 5°-10°, between surfaces to minimize thepossibility of the formation of voids in the resulting bonding layer dueto growth irregularities, which could make access of oxidant difficultas the growing ceramic bond layer comes in contact with the adjacentceramic.

As discussed earlier in this Application, a body of at least one fillermaterial can be placed between the ceramic bodies to be joined. Theoxidation reaction product grows into and embeds this body of fillermaterial. The growth of oxidation reaction product into the fillermaterial forms the bond between the ceramic bodies to be joined. Thefiller should have sufficient permeability to permit growth of oxidationreaction product thereinto.

In addition, the body of filler material should be sufficientlypermeable to permit the oxidant to contact the molten parent metal atthe surface of the body or bodies from which the oxidation reactionproduct growth originates. This is especially important when the oxidantutilized comprises a vapor-phase oxidant. When a body of filler materialis employed, a ceramic composite bond is formed between the ceramic (orceramic composite) bodies to be bonded together.

In another embodiment of the instant invention, the filler material mayalso be placed at least partially within the surface porosity of atleast one of the ceramic (or ceramic composite) bodies to be bondedtogether. This embodiment would permit the formation of a ceramiccomposite bond which extends into (i.e., past the surface of) at leastone of the bodies being bonded together.

For bonding, the assembled ceramic bodies as illustrated in FIGS. 1 and2 are heated in the presence of a suitable oxidant at a temperatureabove the melting point of the residual metal in the first ceramic bodybut below the melting point of the oxidation reaction product. Moltenmetal accessible from the bonding surface (the surface of ceramic body 2or 2' which faces the corresponding surface of ceramic body 4 or 4') isoxidized on contact with the oxidant, and then growth of the oxidationreaction product is induced as described above so as to form a bondinglayer of sufficient thickness. A strong bond can be achieved even withrelatively thin bonding layers, and thus it may be unnecessary, and insome cases undesirable, to permit extensive growth of the bonding layer.

Any one of various parent metals, e.g., aluminum, titanium, tin,zirconium, hafnium, or silicon, can be used in the practice of theinvention, although the invention is described herein with particularreference to aluminum. Also, the oxidation reaction product may be anoxide, nitride, or carbide, depending on the choice of oxidant. When afirst ceramic body is to be bonded to another first ceramic body, thetwo bodies may be of the same or different composition, and if themetals in both ceramic bodies are derived from the same parent metal,the interconnected metal still may differ with respect to the purity,grade or alloy composition.

Ceramic products of other types useful as a second ceramic body whichcan be bonded to a first ceramic body include densified ceramic powders,e.g. a metal oxide, boride, carbide, or nitride which have been pressedand sintered or otherwise processed by conventional methods.

The assembled ceramic bodies to be bonded are heated above the meltingpoint of the residual metal (but below the melting point of theoxidation reaction product to be formed), and an appropriate temperaturewithin this range is maintained for a period sufficient for a bondinglayer of the required thickness to grow. The operable and preferredtemperature ranges vary depending on the metal, dopant(s) used, time,and oxidant. In the case of a molten aluminum parent metal and air asthe oxidant, the reaction temperature may be from about 850° C. to about1450° C., or preferably from about 900° C. to about 1350° C. In thissystem, and particularly in the case in which magnesium and one or moreof the Group IV-A elements, silicon, germanium, tin, and lead arealloyed with the aluminum to act as dopants, a heating time at theselected temperature of only a few hours, e.g., about five hours atabout 1100° C., usually is sufficient to produce a strong bond about0.02 mm or more thick between two ceramic bodies.

When an oxidizing atmosphere is utilized as the oxidant, the oxidizingatmosphere is provided by a vapor-phase oxidant, i.e., a vaporized ornormally gaseous material. For example, oxygen or gas mixturescontaining oxygen (including air) are desirable vapor-phase oxidants, asin the case where a molten aluminum parent metal is to be oxidized toform an alumina reaction product, with air usually being preferred forobvious reasons of economy. The flow rate Df the vapor-phase oxidantshould be sufficient to assure good metal/oxidant contact between theassembled ceramic bodies.

The molten metal consumed in the formation of the bonding layer iscarried within channels of at least the first ceramic body, and themetal channels have open access to the surface of the ceramic. Inproducing the first ceramic body, interconnected metal will remain inthe structure if the growth process is stopped prior to or just at thedepletion of the pool of molten metal that provides the parent metal forthe reaction. If the growth process is continued beyond this point,interconnected metal within the ceramic body is drawn to the surface toform additional polycrystalline oxidation reaction product growth at theinterface with the oxidant, thereby resulting in interconnected porosityin the vacated metal channels. Thus, the first ceramic body used in theprocess of this invention is one which has been made without substantialdepletion of its metal content, by suitable control of process time andtemperature.

Inasmuch as the first ceramic body contains channels of interconnectedmetal, oxidation of the molten metal and growth of the oxidation productcan be expected to occur not only at the bonding surface, but at allfree (exposed) surfaces of the body, as well as on exposed surfaces ofany additional parent metal being used (as described below in connectionwith FIG. 2) to augment the interconnected metal of the first ceramicbody. Growth of oxidation reaction product can be confined to thesurface(s) to be bonded by applying a barrier means to the othersurfaces. As described in U.S. Pat. No. 4,923,832 filed May 8, 1986, andassigned to the same assignee, a suitable barrier means inhibits growthor development of the oxidation reaction product to within definedboundaries or zones. Suitable barrier means may be a compound, element,composition, mixture or the like, which, under the process conditions ofthis invention, maintains some integrity, is not volatile, and may bepermeable or impermeable to the oxidant while being capable of locallyinhibiting, poisoning, stopping, interfering with, preventing, or thelike, growth of the oxidation reaction product. Suitable barriers foruse with aluminum parent metal using air as an oxidant include calciumsulfate (plaster of Paris), calcium silicate, Portland cement,tricalcium phosphate, and mixtures thereof, which typically are appliedas a slurry or paste to the surface of the ceramic body and parent metalas shown in the drawings. These barrier means are well suited forconfining or preventing growth of alumina oxidation reaction productfrom molten aluminum in air, and thereby may used to prevent growthother than toward the bonding zone.

FIG. 1 shows a barrier means 6 (partially broken away for clarity ofillustration) which is applied to all free or exposed surfaces of firstceramic body 2, so that oxidation of residual metal and growth ofoxidation reaction product from first ceramic body 2 is confined to thebonding surface of ceramic body 2, i.e., the surface thereof facing orabutting a corresponding surface of second ceramic body 4.

In the present method, wherein the molten metal required to produce thebonding layer is supplied by the first ceramic body, this body may havebeen formed originally under such process conditions that it is depletedof interconnected metal, and consequently is porous or at leastpartially porous. The first ceramic body can be augmented with parentmetal by contacting an exposed surface of the ceramic with an additionalbody of parent metal, which may be the same or different from that usedin producing the original first ceramic body. This technique isillustrated in FIG. 2 in which a parent metal body 8 is positionedadjacent to a free surface of the first body of ceramic 2', i.e.,adjacent to a surface thereof other than a bonding surface which facesor abuts a surface of second ceramic body 4'. All of the surfaces offirst ceramic body 2', except its bonding surface and the portion of itssurface in contact with parent metal body 8, are covered by a barriermeans 6', which is also applied to all of the exposed surfaces of parentmetal body 8. The bonding process is carried out as described above, andmolten parent metal, as it reacts to form oxidation reaction product, istransported therethrough and to the bonding surface where oxidationreaction product then forms as the bonding layer. Even when the firstceramic body contains interconnected metal, additional parent metal maybe supplied to prevent the generation of porosity in the body as metalis drawn to the surface to form the bond layer.

EXAMPLE

In order to show the utility of this invention, two 4.8 mm thick

ceramic plates were bonded end-to-end at surfaces measuring 4.8 mm×7.9mm. The plates both originated from a single piece of alumina ceramicwhich had been formed by the oxidation reaction of molten aluminumparent metal (aluminum alloy 5052 containing nominally 2.4% magnesium),externally doped with a thin layer of SiO₂ and exposed for 120 hours at1175° C. to form the alumina ceramic. These ceramic bodies containedinterconnected aluminum in dispersed channels which extended to thesurfaces.

The ceramic plates were positioned end-to-end and laid on edge in a highpurity alumina boat where they were heated at 1175° C. for five hours inflowing air. When cooled, the total weight of the assembly was found tohave increased by 2.4%. The plates were firmly bonded together,end-to-end, by a 0.018-mm-thick layer of newly grown alumina ceramic. Inaddition, a 0.05-mm-thick layer of alumina ceramic had also grown on theother exposed surfaces including the surfaces in contact with the boatwhich was also strongly bonded to the ceramic plates. The new growth hada finer microstructure than that of the original ceramic plates, withfinely dispersed aluminum in evidence therein. An attempt was made torecover the bonded plates by hammer blows to the boat to break it away.All of the bond zones remained intact, indicating a high degree ofbonding both between the ceramic plates and from the plates to the highpurity alumina boat.

We claim:
 1. A bonded body, comprising a first portion of ceramic comprising a three-dimensionally interconnected ceramic matrix comprising oxidation reaction product, and at least one second portion of ceramic bonded to said first portion of ceramic by at least one bonding zone, said at least one bonding zone comprising oxidation reaction product and at least some at least partially interconnected metallic phase.
 2. A bonded body, comprising at least two ceramic portions, each of said at least two ceramic portions comprising three-dimensionally interconnected ceramic matrix comprising oxidation reaction product, and at least one bonding zone which integrally bonds said at least two ceramic portions to one another, said at least one bonding zone comprising oxidation reaction product and at least some at least partially interconnected metallic phase.
 3. A bonded body, comprising at least two ceramic composite portions, each of said ceramic composite portions comprising three-dimensionally interconnected ceramic matrix and at least one filler, and at least one bonding zone which integrally bonds said at least two ceramic portions to one another, said bonding zone comprising oxidation reaction product and at least some at least partially interconnected metallic phase.
 4. The bonded body of claim 2, wherein at least one but less than all of said at least two ceramic portions further includes at least one filler.
 5. The bonded body of claim 1, wherein said at least one bonding zone further comprises at least one filler in at least a portion thereof.
 6. The bonded body of claim 2, wherein said at least one bonding zone further comprises at least one filler in at least a portion thereof.
 7. The bonded body of claim 3, wherein said at least one bonding zone further comprises at least one filler in at least a portion thereof.
 8. The bonded body of claim 4, wherein said at least one bonding zone further comprises at least one filler in at least a portion thereof.
 9. The bonded body of claim 1, wherein said first portion of ceramic further comprises at least one of interconnected metal and interconnected porosity in at least a portion thereof.
 10. The bonded body of claim 2, wherein said at least two ceramic portions further comprise at least one of interconnected metal an d interconnected porosity in at least a portion thereof.
 11. The bonded body of claim 3, wherein said at least two ceramic portions further comprise at least one of interconnected metal and interconnected porosity in at least a portion thereof.
 12. The bonded body of claim 1, wherein said at least one bonding zone is distinguishable from said first portion of ceramic and said at least one second portion of ceramic in at least one of composition, microstructure and properties.
 13. The bonded body of claim 2, wherein said at least one bonding zone is distinguishable from said at least two ceramic portions in at least one of composition, microstructure and properties.
 14. The bonded body of claim 3, wherein said at least one bonding zone is distinguishable from said at least two ceramic composite portions in at least one of composition, microstructure and properties.
 15. The bonded body of claim 1, wherein said at least partially interconnected metallic phase comprises at least one metal selected from the group consisting of aluminum, silicon, zirconium, tin, hafnium and titanium.
 16. The bonded body of claim 2, wherein said at least partially interconnected metallic phase comprises at least one metal selected from the group consisting of aluminum, silicon, zirconium, tin, hafnium and titanium.
 17. The bonded body of claim 3, wherein said at least partially interconnected metallic phase comprises at least one metal selected from the group consisting of aluminum, silicon, zirconium, tin, hafnium and titanium.
 18. The bonded body of claim 3, wherein said at least one filler comprises at least one material selected from the group consisting of aluminum oxide, silicon carbide, silicon aluminum oxynitride, zirconium oxide, zirconium boride, titanium nitride, barium titanate, boron nitride and silicon nitride.
 19. The bonded body of claim 4, wherein said at least one filler comprises at least one material selected from the group consisting of aluminum oxide, silicon carbide, silicon aluminum oxynitride, zirconium oxide, zirconium boride, titanium nitride, barium titanate, boron nitride and silicon nitride.
 20. A bonded body comprising an array of a plurality of ceramic portions bonded by a plurality f bonding zones, said bonding zones comprising oxidation reaction product and at least some at least partially interconnected metallic phase, wherein at least every other ceramic portion within said array comprises a three-dimensionally interconnected ceramic matrix comprising oxidation reaction product.
 21. The bonded body of claim 1, wherein said first portion of ceramic and said at least one second portion o ceramic are positioned relative to one another by said bonding zone such that a small angle exists between the surfaces of each of said first portion of ceramic and said at least one second portion of ceramic.
 22. The bonded body of claim 2, wherein said at least two ceramic portions are positioned relative to one another by said bonding zone such that a small angle exists between the surfaces of each of said at least two ceramic portions.
 23. The bonded body of claim 3, wherein said at least two ceramic composite portions are positioned relative to one another by said bonding zone such that a small angle exists between the surfaces of each of said at least two ceramic composite portions.
 24. The bonded body of claim 9, wherein the composition of the interconnected metal is substantially the same as the composition of the at least partially interconnected metallic phase.
 25. The bonded body of claim 10, wherein the composition of the interconnected metal is substantially the same as the composition of the at least partially interconnected metallic phase.
 26. The bonded body of claim 9, wherein the composition of the interconnected metal is different from the composition of the at least partially interconnected metallic phase.
 27. The bonded body of claim 10, wherein the composition of the interconnected metal is different from the composition of the at least partially interconnected metallic phase.
 28. The bonded body of claim 1, wherein said at least one second portion of ceramic comprises at least one body formed by densification of at least one ceramic powder. 